1. Technical Field of the Invention
The invention relates generally to the field of fiber optics and more particularly to electro-optical modulators.
2. Description of Related Art
Electro-optical modulators in a recent trend are moving toward conjugating smaller dimensions and lower driving voltages to enable system vendors to design their opto-electronic boards, e.g. transponders, by adopting smaller footprints, with inexpensive and low power electronic components. Conventional solutions employ three design techniques for reducing a RF driving voltage in a Lithium Niobate (“LN”) travelling wave modulator: (1) adopting a longer RF electrode, and thus resulting in a longer, instead of a shorter, chip; (2) decreasing the buffer-layer thickness, resulting in an overall worsening of the performances; and (3) decreasing the gap width between hot and ground electrodes of the CoPlanar transmission line, resulting once more in an overall worsening of the performances.
Each of these three techniques has shortcomings. First, if the RF electrode is lengthened, the chip and package dimensions in turn must be lengthened. In addition, the total electrical losses increase, thereby decreasing the electro-optical bandwidth. Thus, such a solution is not suitable for designing metro application-oriented components. Second, if the buffer-layer thickness is decreased, the electro-optical bandwidth also decreases due to the total electrical losses increasing and microwave effective refractive index being more difficult to be matched to the optic. Furthermore, the characteristic impedance of the microwave lines becomes worse, rendering more difficult to have a good electrical return loss vs. frequency profile. A good impedance matching between the driver output port and the modulator input port is desirable for a correct working of the entire transmission chain. A typical specification for the electrical return loss of the modulator (S11 parameter) is <−10 dB over the whole operating bandwidth. Third, if the gap width is decreased between hot and ground electrodes of the CoPlanar transmission line, the electro-optical bandwidth decreases due to the increasing in total electrical losses, while the characteristic impedance of the microwave lines becomes worse. A graphical diagram 100 in FIG. 1 illustrates typical profiles of S11 parameters versus frequency for a standard x-cut modulator, which shows values well below the required specification, i.e. −10 dB.
Accordingly, there is a need to design a system for reducing the electrical return loss of Lithium Niobate traveling wave electro-optical modulators with low characteristic impedance.